Natural gas may be supplied to engines as fuel and comprises several different gases including methane and others, such as, ethane, propane, butane, carbon dioxide, oxygen, hydrogen, and nitrogen. Natural gas also may include water and hydrogen sulfide. Natural gas may also include large or unsaturated hydrocarbons, which are hydrocarbons with double or triple covalent bonds between adjacent carbon atoms.
Each of the gases comprising natural gas may have different combustion properties which correlate to the knock tendency or knock resistance of that gas during combustion in an engine. A methane number (MN) is an index number for classifying gases according to their knock resistance. For example, methane gas has a high knock resistance and, therefore, has a methane number of 100, while hydrogen gas has a low knock resistance because it combusts fasters than methane gas and, therefore, has a methane number of 0.
The methane number for a given natural gas affects the fueling of an internal combustion engine. For example, the valve timing and injection timing may be adjusted according to the methane number of the gas supplied to the engine in order to prevent knocking in the combustion chambers.